The present invention relates generally to a three-dimension curved shape forming method and apparatus, and more particularly to a polishing method and apparatus. The present invention is suitable, for example, for a curved shape formation and surface processing for an optical element that are required to have a high surface precision on its surface, such as freely curved mirrors and lenses.
Optical elements for use with optical communications, such as a mirror and a lens, are required to have a higher surface precision with the recent high speed and large capacity communications. In particular, a mirror used for a variable optical dispersion compensator in the Dense Wavelength Division Multiplexing (“DWDM”) has such a small area as 10 mm×several millimeters and a complicated free-form surface shape, and needs a very high surface precision. Several variable optical dispersion compensators have already been proposed for the DWDM (see, for example, International Application Domestic Publication No. 2002-514323 and Yuichi Kawabata, Noriaki Mitamura, and Hideki Isono, “VIPA dispersion compensator for 40 Gbps WDM system”, Electronic Material, Kogyo Chosakai Publishing Inc., Nov. 1, 2001, Vol. 40, No. 11, pp 67-69.
In order to manufacture an optical element having such a complicated free-form surface shape, the prior art uses a three-dimensional processing unit having five or six degrees of freedom to manufacture a mold for the optical element, then a molding compound, such as resin and glass, is molded into a mirror shape, and finally a mirror surface is generated by evaporating aluminum or gold onto a necessary surface. As a method that forms a target surface shape on an optical element, such as a lens and a rod mirror, Japanese Patent Application, Publication No. 2000-84818 discloses a method that pressurizes plural actuators (pressurizers) for polishing. Other prior art includes, for example, Japanese Patent Application, Publication No. 10-118917.
However, a method that uses the three-dimensional processing unit and resin molding transfers a trace of tool on a mold's free-form surface part, onto a free-form surface part on a resin molded article, and lowers the surface precision. On the other hand, a hand lap (i.e., a fine polishing method that is performed by an operator's hands) is one measure to remove the trace of tool in advance. Nevertheless, the hand lap destroys a shape optimized in the processing unit, and cannot reconcile the form accuracy with the surface precision.
A method disclosed in Japanese Patent Application, Publication No. 2000-84818 forms a shape during polishing and does not generate problems associated with the three-dimensional processing unit. Originally, the load applying approach by an external mechanism during polishing is usually used for a wafer flattening process in the chemical mechanical polishing (“CMP”), and the methodology is common in that both control a polishing amount distribution on a work's polished surface using an external mechanism, although they have different objects, such as shaping and flattening. The conventional load applying method including the flattening polishing process can be classified into two types—a method (shown in FIG. 12) that provides a processing head that holds a work with fine holes in its work holding surface, connects these holes with an air supply source, and applies the load to the work through the air pressure control; and a method (shown in FIG. 13) that provides a processing head with plural actuators, such as an air cylinder and piezoelectric element, and directly applies the load to the work's back surface.
The fundamental concept of these methods is to apply the load through an external mechanism (such as the above air pressurizer and actuator) to a location at which the polishing amount is to increase and to locally improve the contact pressure and polishing speed. Nevertheless, these methods can control the pressure only around the air supply hole in the method shown in FIG. 12 and around the actuator's contact pressure point in the method shown in FIG. 13. In other words, these methods apply the high point load only near the application point of the load, and cannot control the load at other positions where there is no mechanism. Therefore, the high form accuracy requires many application points. For a relatively large work, such as a wafer, a predetermined number of application points (for example, several tens of application points or air supply holes are enough for the processing head in the method shown in FIG. 12) may be provided. However, only several points can be provided for a small work such as an optical element, for example, having 10 mm square or smaller. Another problem is that the method that provides application points directly on the work's back surface and controls the contact pressure against the work by plural point loads causes an excessive high pressure difference between the vicinity of the application point where the contact pressure is locally high and the other positions, and results in irregularities corresponding to the application points on the resultant polished surface. In addition, when the work has a thickness of 1 mm or smaller, the work itself may possibly get damaged.